The search for new or improved function of biological molecules is a challenging endeavor. Methods for generating and screening libraries of variant proteins have been developed, e.g., to make and identify proteins exhibiting a desired property (see, for example, Stemmer, W. P. (1994) “Rapid evolution of a protein in vitro by DNA shuffling” Nature 370:389-391). Some methods result in the generation of a library containing few functional molecules (see, for example, Ostermeier (1999) “A combinatorial approach to hybrid enzymes independent of DNA homology” Nature Biotech 17:1205). Limitations in screening capacity can make it difficult to find such functional molecules. Accordingly, a need exists for methods of producing libraries of biomolecules that are enriched for functional biomolecules.